winch snatch block storage question

[wldrbob]

yes canadian 135

 

[greenjeepster]

Okay so I understand the usefulness of a snatch block if you are winching somebody else out, but how do you use it to improve your own pulling power if it is your own winch truck that was stuck?

For example: You are cutting firewood and sink in up to the frame. You have 2 cords of wood on the bed weighing 8000 lbs. Because you are sitting on the frame you have 100% resistance. So a combined weight of 21000 lbs. You have a 10000 lb winch and you have a 200 year old red oak within reach of the front winch to use as an anchor. What do you do?

 

[stumps]

Okay so I understand the usefulness of a snatch block if you are winching somebody else out, but how do you use it to improve your own pulling power if it is your own winch truck that was stuck?

At the risk of repeating myself, I will repeat myself.

Assuming that your snatch blocks have pulleys in them with little friction, the tension in the winch cable is the same from one end to the other. This is something you must remember.

If you hook a snatch block to a tree, and run the cable back to the winch truck's bumper, you now have two cables pulling on both the truck, and the tree. Since each cable has the same tension in it, you now have twice the pulling power.

If you run the cable from the snatch block to a tree, and then back to a snatch block on your bumper, and then back to the tree, you now have three cables pulling on both the tree, and the truck. Since each cable has the same tension in it, you now have three times the pulling power of a single cable.

Count the cables that are pulling on what you want to move, and that is the multiplier you get from using the snatch blocks.

For example: You are cutting firewood and sink in up to the frame. You have 2 cords of wood on the bed weighing 8000 lbs. Because you are sitting on the frame you have 100% resistance. So a combined weight of 21000 lbs. You have a 10000 lb winch and you have a 200 year old red oak within reach of the front winch to use as an anchor. What do you do?

Is this what has happened to your truck?

First thing to note, is friction and mud don't work like you think. Frictional force resists the direction of motion, and it is equal to the friction constant, times the weight of the truck. On ice, the friction constant could be so small that the frictional force is negligible, and you slip and slide easily, or you could be with locked up brakes, on pavement where the friction constant is well above one, and the force needed to pull the truck could vastly exceed the weight of the truck.

Mud is a totally different animal. If your tires are sunk down in the mud so deep that there is no rolling up out of the holes, you have transformed your truck into a plow. If you pull straight off of the bumper, your wheels will have to plow the dirt for you to move forward, and that could take astronomical amounts of force from the winch.... 5 or ten times the weight of the truck... Most of the time trying this trick will break something.

So, don't pull straight forward! Either jack the truck wheels out of the holes, and put a temporary road under the wheels (planks, logs, etc...), or arrange your winch cable to go over a short chunk of pole dug into the ground in front of the truck. The pole changes the winch's pull from plowing to lifting.

-Chuck

 

[greenjeepster]

Now from what I am seeing in the recovery manual running the cable through the snatch block at the tree and back to the truck provides no additional pulling power. The snatch block pulley must move in order multiply the pull weight. If the tree was another truck that you were pulling toward you the cable would be feeding through the pulley and you would double the pulling power of the winch because the snatch block is winding in at half the speed the winch is. You do not get the same benefit with the tree because it is stationary. Running the cable through a snatch block attached to the tree would be the same as just taking the winch cable and going around the back of the tree with it and back to the truck... Straighten the cable out and you have a straight cable.

Think of it like if you were pulling a stuck deuce with another deuce. It would not matter if you had one tow chain attached between the trucks or 50... you still only have the pulling power of the truck.

Now with a second snatch block attached to the truck and the cable run back to the tree you would multiply the pulling power because the snatch block at the truck is doing the same thing as a single attached to another truck.

I understand the friction the mud is adding to the pull weight. I am leaving it out because in the given situation we are not going to be able to calculate the actual friction weigh accurately anyway. So we might as well leave that part out and go on what we know.

So if you are going to have to jack the truck anyway; how is the winch going to be useful in this situation? I can jack a truck without a winch, put staging under it and drive it out just as easily.

Under what scenario would a winch be useful for self recovery when the pull weight is at or exceeds the weight of the truck?

Oh to answer your question: My truck is safe and sound in the driveway. The scenario is hypothetical. I am weighing the usefulness of a winch for my personal use. I do not off road recreationally; the truck is strictly a work vehicle when off road. I don't have a second truck to recover so recovery is not an issue for me.

 

[mudguppy]

Now from what I am seeing in the recovery manual running the cable through the snatch block at the tree and back to the truck provides no additional pulling power. The snatch block pulley must move in order multiply the pull weight. ....

this is incorrect. go watch a crane with a multiple sheave tackle block - it's the same thing; the tip of the crane boom block is your truck and the tackle block is your snatch block.

 

[KsM715]

It's physics. If I take and A-frame, and a 2000lb block of iron, I can lift it with my scrawny little 180lbs by using enough steel cable and putting enough pulleys between the A-frame and block of iron. Do a google search for block and tackle and how it works. I'm sure you can find someone that can explain it much bettert than I can. (that FM 20-22 does it pretty well though)

 

[greenjeepster]

this is incorrect. go watch a crane with a multiple sheave tackle block - it's the same thing; the tip of the crane boom block is your truck and the tackle block is your snatch block.

We are not talking about multiple sheave tackle. We are talking a single block simple tackle at the tree. Look at page 25 of the Recovery Manual. It has the ratios listed; for simple tackle it lists a 1:1. ratio. The only advantage of attaching a block to the tree and running the cable through it would be to get the cable out far enough in order to maximize the 10000 lb pull weight. i.e. your anchor point is too close to the winch.

It will floor me if all of you winching pros on here have this all wrong.

b. Running Block. A block that is attached to
the load and moves with the load is classified as a
running block. A running block will always gain
a mechanical advantage, and its sheave functions
as a second-class lever.

c. Floating Block. A block used with a tow cable
to allow the pull of the cable to aline with the
source of power is known as a floating block. A
floating block provides no mechanical advantage.
It allows the pull to be equally distributed to both tow attachments.

For self recovery you will only gain a mechanical advantage if you are using two blocks, one at the anchor and one at the truck.

 

[mudguppy]

it doesn't matter. then go look at a crane with a single-sheave tackle block. the static end of the line goes back to the boom, therefore, the hoist line is seeing half of what the tackle block is seeing - it's a 2:1 advantage.

say you're lifting 20k lbs with the crane. with single-sheave block, the hoist line is only seeing half of the load (10k lbs), and the static end tied back to the boom is seeing the other half (10k lbs).

the block itself is lifting all 20k lbs as is the boom. make sense?

 

[reb87]

The mechanical advantage (MA) of any
simple tackle rigging is equal to the number of
lines supporting the load, or number of lines that
become shorter as power is applied to the winch,
whether the lines are attached directly or indirectly
through a block (fig 20)

Your example is like the 2:1 in figure 20. The 1;1 is your winch going out thru a stationary block and back to something you are pulling towards the pulley(not the truck)

In the 1:1 the distance from the winch to the pulley is not getting shorter. In your example the distance between the truck and pulley is getting shorter(both cables are getting shorter)

 

[KsM715]

Here I fixed the picture. I scratched out everthing but the single block with one pulley.
Does this help.

To confuse you more, your forward movement is now half of your winch line speed.

Edit: there are no floating blocks in the figure we are using for reference. only running blocks. Even though the block is attatched to the tree it is still a running block. Its just not running anywhere.

 

[sdb3023]

^As long as the winch is on the truck that is getting winched out, this is true, if your going to a truck next to the stuck truck then there is no Mech Advan.

 

[mudguppy]

...For self recovery you will only gain a mechanical advantage if you are using two blocks, one at the anchor and one at the truck.

that's not what the info you just posted says - you don't need two blocks to gain a mechanical advantage. you can have a single block and gain a 2:1 pulling advantage.

running block example - pulling out a stuck truck; attach the block to the truck and the static end back to either the truck or an anchor point close to it. now the truck winch will pull 10k lbs but the block will be applying 20k lbs because of the advantage.

running block example - pulling self out; attach the block to an anchor point and the static end back to self. now the truck winch will pull 10k lbs, however, the block allows for a 2:1 advantage because the static end is moving with the truck.

floating block example - pulling out a stuck truck; attach the block to an anchor point (to pull around a corner, for example) and run the cable through the block and to the stuck truck. the truck winch will pull 10k lbs and the static end attached to the truck is pulling the same 10k lbs. therefore, the truck only sees 10k lbs. also, the block and anchor point may experience up to 20k lbs, depending on angle of pull (if close to 180°).

 

[greenjeepster]

Okay; I got it: If the "tree" was a load that was moving and had 20000 lbs of pressure exerted on it; the same load stationary also has 20000 lbs exerted on it so the 20000 lbs is transferred to the truck.

But only if the winch is on the truck being pulled. It is pulling both sides at the same time there by dividing the wind in speed by 2 and increasing the toque by 2.

 

[KsM715]

Whew glad thats over I was afraid we were getting way off track of the OP's original intentions and gonna start to draw fire from the Mods.

Back to the OP's storage ? I'd like to see any pics anyone has of added storage to their trucks.

 

[mudguppy]

i have an extra d-side tool box that i intend to install behind the stock box. i'll store fluids and parts in one, tools and recovery aids (including snatch blocks) in the other.

actually, i need another step - i have the box and supports, but the step 'fell off' during transit. anyone have a step????

 

[stumps]

Now from what I am seeing in the recovery manual running the cable through the snatch block at the tree and back to the truck provides no additional pulling power. The snatch block pulley must move in order multiply the pull weight.

Ok, we need some basic physics learning here!

Rope has little stiffness, so for our purposes, you can only have tension in it, and let's forget about the winch for a minute.

Let's assume our friends Thing 1 and Thing 2 are playing tug-a-war. Each is holding his side of the rope, and is pulling as hard as he (it?) can.

Question: How can we tell if Thing 1 is pulling as hard as Thing 2?
Answer: The rope isn't moving.

Question: How can we tell if Thing 1 is pulling harder than Thing 2?
Answer: The rope is moving towards Thing 1.

Question: How can we tell if Thing 2 is pulling harder than Thing 1?
Answer: The rope is moving towards Thing 2.

Now, let's play tug-a-war with the tree.

Suppose we attach a snatch block to the tree, and run a rope through the snatch block, and Thing 1 and Thing 2 each grab a free end of the rope and pull as hard as they can.

Question: How can we tell if Thing 1 is pulling just as hard as Thing 2?
Answer: The rope isn't moving through the pulley.

Question: How can we tell if Thing 1 is pulling harder than Thing 2?
Answer: The rope is moving towards Thing 1.

Question: How can we tell if Thing 2 is pulling harder than Thing 1?
Answer: The rope is moving towards Thing 2.

Question: If Thing 1 and Thing 2 are pulling equally hard: What is the total force pulling on the tree?
Answer: The force on the tree is always Thing 1 + Thing 2. If Thing 1 = Thing 2, the total force is 2 x Thing 1.
...

Ok, with that out of the way, suppose the rope was tied to the bumper on one end, and on the other end it was connected to a winch (also attached to the bumper). As the winch starts to draw up the tension, the rope will move through the snatch block, and the pulley will rotate.

Question: Why does the pulley rotate?
Answer: The winch side of the rope is pulling harder than the side tied to the bumper.

Let the winch keep pulling until the rope is taut, and the pulley in the snatch block stops turning.

Question: If the tension applied by the winch is equal to Thing 1, and the pulley isn't turning, how hard is the snatch block pulling on the tree?

Answer: See the above example. If the pulley isn't turning, it is because the tension in both sides of the rope is equal, and since the tension in one side is equal to Thing 1, the tension in the other side is also equal to Thing 1. Two ropes pulling on the snatch block each with tension Thing 1 in it, will result in 2 x Thing 1 pulling on the tree.

Question: If the truck is pulling on the tree with a force of 2 x Thing 1, how hard is the tree pulling on the truck?

-Chuck

 

[KsM715]

greenjeepster, If you get bored and want to really get a headache ( or learn something) read this thread, http://www.steelsoldiers.com/deuce/33201-more-winching-ops-questions.html. On page 2 you come to a picture posted by cranetruck, well I'll post it here, study it and try to figure it out, then go read the thread. I thought no way in the world is that possible, but that why I love this site. I learn something new every day.

Dont take this the wrong way. This is not meant as a dig on you. I just thought you might enjoy it. FM 5-125 rigging has alot of good info also.

 

[greenjeepster]

greenjeepster, If you get bored and want to really get a headache ( or learn something) read this thread, http://www.steelsoldiers.com/deuce/33201-more-winching-ops-questions.html.

That actually makes perfect sense to me. At first it looks like a good way to pull the truck in half, but you are gaining slack to the front from between the blocks at they pull closer together.

I think it would be easier to wrap heavy rope around the rear tandems though.

 

[KsM715]

You read the thread didnt you?

 

[Bighurt]

Question: If the truck is pulling on the tree with a force of 2 x Thing 1, how hard is the tree pulling on the truck?

That depends one weather thing 1 is moving towards the tree.